See -ful in Webster's New World College Dictionary
full of, characterized by, having: joyful, painful
having the qualities of: masterful
having the ability or tendency to, apt to: helpful, forgetful
the quantity that fills or would fill: the plural is formed by adding -s to the resulting word [teaspoonfuls, handfuls] or, sometimes, by adding -s- between the noun stem and the suffix [cupsful]